This invention relates to a method and apparatus for measuring the size of a wire rod with a laser beam without making any contact with the wire rod. More particularly, this invention relates to a method and apparatus of the kind described above which can measure the size of a pin or the like having an outer diameter smaller than the diameter of a laser beam spot.
Methods for measuring the size (the outer diameter) of a pin, a wire rod or the like by the use of a laser beam are disclosed in, for example, "Illustrations of Applications of Laser Beams" described in "Electronics Review, January-July Number, 1980".
According to one of the prior art methods disclosed in this paper, an object such as a pin or a wire rod is scanned with a parallel laser beam, and the period of interception of the laser beam by the object is measured so as to calculate the size (the outer diameter) of the object. According to another method disclosed in the paper, a laser beam not scanning such an object is directed toward the object, and the distance between bright spots formed on a screen by the diffraction of the laser beam is measured so as to calculate the size (the outer diameter) of the object.
The former prior art method disclosed in the paper for measuring the outer diameter of a wire rod by scanning with a laser beam will be described in detail with reference to FIGS. 1(A), 1(B) and FIGS. 2(A), 2(B), so that the present invention can be more clearly understood.
FIGS. 1(A), 1(B) and FIGS. 2(A), 2(B) show the relation between a laser beam spot 8 and a pin 6 whose outer diameter is unknown and to be measured and show also the relation between time and an output voltage of a beam receiving element when the outer diameter of the pin 6 to be measured is larger and smaller than the diameter of the laser beam spot 8 respectively.
Referring to FIGS. 1(B) and 2(A), the laser beam spot 8 is controlled to traverse at a predetermined speed from, for example, the left-hand side toward the right-hand side in FIG. 1(A) by means (now shown) so as to scan the pin 6. A beam receiving element (not shown) is located behind the pin 6 through an interposed optical system (not shown) to receive the laser beam spot 8 during scanning. When the laser beam spot 8 being traversed for scanning is intercepted by the pin 6, the output voltage of the beam receiving element decreases, and, when the laser beam spot 8 moves away from the pin 6, the output voltage of the beam receiving element increases. As a result, when the outer diameter of the pin 6 is larger than the diameter of the laser beam spot 8, the output voltage E of the beam receiving element decreases progressively with lapse of time of scanning with the laser beam spot 8 until the output voltage E drops to its "O" potential level as shown in FIG. 1(B), because the laser beam spot 8 is entirely intercepted by the pin 6. Then, the output voltage E of the beam receiving element increases progressively. Therefore, by measuring the period of time during which the output voltage E of the beam receiving element is lower than a specific value, for example, 1/2 of its highest level Eo, the outer diameter of the pin 6 can be calculated on the basis of the measured period of time.
On the other hand, when the outer diameter of the pin 6 to be measured is smaller than the diameter of the laser beam spot 8, the entirety of the laser beam spot 8 is not intercepted by the pin 6. Therefore, the output voltage E of the beam receiving element does not decrease to a level lower than a certain level. That is, the output voltage E will not decrease to a level lower than the aforementioned specific voltage E.sub.0 /2 as shown in FIG. 2(B). In such a case, the outer diameter of the pin 6 cannot be measured.
As discussed above, in the prior art method for measuring the outer diameter of a pin by scanning with a laser beam spot, the outer diameter of the pin can be measured within an allowable measurable range of an optical system used for measurement when the outer diameter of the pin is larger than the diameter of the laser beam spot. However, it has been unable to measure the outer diameter of the pin when the outer diameter of the pin is smaller than the diameter of the laser beam spot. Also, it has been unable to measure the outer diameter of the pin with satisfactory accuracy even when the level of the specific voltage to be compared with the output voltage of the beam receiving element is selected to permit measurement of the outer diameter of the pin.